A patent document 1 discloses an internal combustion engine which includes a fuel injection device including a port injection fuel injection valve arranged to inject fuel to an intake port, and a cylinder injection fuel injection valve arranged to inject the fuel directly into a combustion chamber, and which is arranged to appropriately switch these in accordance with an engine driving state. In this patent document 1, a driving region determined by the load and the rotation speed of the engine is divided into three regions of a stratified lean combustion region of a low speed and low load side, a homogeneous lean combustion region of a middle speed and middle load, and a homogeneous stoichiometric combustion region of a high speed and high load side. In the stratified lean combustion region, the cylinder injection is performed by using the cylinder injection fuel injection valve. In the homogeneous lean combustion region, the intake port injection is performed by using the port injection fuel injection valve. In the homogeneous stoichiometric combustion region, the fuel injection is performed by using both of the fuel injection valves.
In this case, the patent document 1 relates to an ignition timing control in which the ignition timing is corrected in accordance with a ratio between the fuel injection amounts of the intake port injection and the cylinder injection. When the variation amount of the appropriate ignition timing according to the variation of the injection amount rate is equal to or greater than a predetermined amount, a width of the variation of the injection amount rate is restricted. That is, for example, even when the target injection amount rate is varied from 0% to 100%, the actual variation of the injection amount rate is performed in a stepped manner.
In the art of the patent document 1, for example, in a case where the fuel injection of the other of the fuel injection valves which is in the stopped state is started in accordance with the variation of the engine driving condition during the driving by the fuel injection from one of the fuel injection valves as described above, the fuel injection of the other of the fuel injection valves is started from the fuel injection of the small amount so as to gradually vary the injection amount rate. However, in the fuel injection valve which is opened in response to the driving pulse signal, a minimum fuel injection amount which can be measured by the driving pulse signal is determined. Accordingly, it is not possible to inject the fuel of the amount which is smaller than the minimum fuel injection amount.
Consequently, even if the injection of the other of the fuel injection valves is started from the extremely small amount during the fuel injection of one of the fuel injection valves, it is not possible to avoid that the fuel injection amount is increased and decreased by the minimum fuel injection amount in the stepped manner. Besides, there is a difference of the transfer delay to the combustion chamber between the intake port injection and the cylinder injection. Accordingly, it is extremely difficult to cancel the above-described stepped increase and decrease of the minimum fuel injection amount, by the injection amount of the one of the fuel injection valves.